The ultrasonic diagnostic apparatus enables observation of the internal structure of a body and the blood flow profile in a non-invasive and simple fashion and is therefore used in a wide variety of medical applications.
Many of the ultrasonic diagnostic apparatuses have multiple display modes. For example, the B mode for displaying the information about the physical state of the internal structure of the biological body in the form of a tomographic image, the color-Doppler (CDI, CFM) mode for obtaining the bloodstream information, and the pulse-Doppler (PWD) mode.
The ultrasonic diagnostic apparatus transmits an ultrasonic beam (pulse wave) into the body of a biological body and performs signal processing on a reflection echo signal received via a probe, thereby obtaining biological information. The pulse width, the wave number and the amplitude of the transmitted pulse wave are different among the respective transmission modes such that the optimum reception process is achieved in each of the display modes.
Generally, in the B mode, a wide-band signal, i.e., a short-pulse wave transmission signal that includes about one to two waves, is used. One of the reasons for this is that the B mode is employed in diagnosis of an organ boundary or diagnosis as to whether or not there is a tumor or polyp which is performed based on the physical state information, and therefore, a wide band signal which is excellent in terms of resolution is suitable to this application. On the other hand, in the color-Doppler mode and the pulse-Doppler mode, a narrow-band transmission pulse, i.e., a long-pulse wave transmission signal that includes about four to eight waves, is used. One of the reasons for this is that these Doppler modes are employed in the case where a plurality of pulses are transmitted to and received from one site and the blood flow profile and the bloodstream spectrum are obtained from the relationships among the phases of respective received waves, and therefore, a signal in a specific frequency band (narrow-band signal) is suitable to this case.
To always improve the S/N ratio in the respective modes, the amplitude of a pulse applied to a piezoelectric vibrator may be increased by increasing the transmission supply voltage.
However, to reduce the effect of ultrasonic energy on a biological body, the energy of the transmission signal transmitted from the ultrasonic probe is limited. The energy of the transmission signal depends on the amplitude and the number of vibrations (frequency) of the signal. Therefore, in the color-Doppler or pulse-Doppler mode in which the transmission signal used has a long pulse wave, it is necessary to decrease the transmission supply voltage as compared to the B mode.
One of the existing solutions to this problem is to change the supply voltage for every scan line and controls the pulse amplitude for every transmission mode. To this end, it is necessary to change the supply voltage with an extremely high rate in order to change the supply voltage for every scan line. This may be accomplished by, for example, a technique disclosed in Patent Document 1.
FIG. 12 shows a conventionally-known transmission unit for controlling the pulse amplitude for every one of the transmission modes. This transmission unit includes transmission circuits of two different types which correspond to the respective transmission modes, and two pullers 51, 53 are respectively coupled to two drive amplifiers 52, 54 which are set to different driving voltages. By selectively switching the drive amplifier 52 and the drive amplifier 54 for every transmission mode, the switching is accomplished within a short period of time. Patent Document 1 describes a use with the combination of the B mode and the continuous-wave Doppler mode, although the same applies to the combination of the B mode and the color-Doppler mode and the combination of the B mode and the pulse-Doppler mode.